1. Field of the Invention
The present invention relates to field effect transistors and particularly to ion-sensitive field effect transistors and a method for producing them.
2. Description of the Related Art
Ion-sensitive field effect transistors (ISFET) serve for measuring ionic concentrations or special substance concentrations in solutions of various compositions and conductivities. Applications of ion-sensitive field effect transistors are, for example, for the continuous detection of concentrations in environment monitoring, in industrial process monitoring, in the food industry, in biochemistry and medical technology, and in laboratory analytics. Here, in particular, a highly precise concentration detection and a minimal long-term drift of the sensor are required, which should further be combined with an acceptable price of the sensor.
It is known that ionic concentrations in aqueous media are measured with glass electrodes both in process measurement technology and in laboratory measurement technology. For technical reasons, particularly because a sufficiently large internal buffer volume combined with an also sufficiently coated internal bleeder electrode and an allowably stable glass membrane as sensor-active component are required, there is no possibility of an efficient miniaturization of the glass electrodes. The internal buffer volume and the coating of the internal bleeder electrode have to satisfy the requirements of the temperature-time load estimated for the sensor. On the other hand, due to the necessary glass membrane thickness, the pH measuring system of the glass electrode measuring chain is a high impedance system, which makes it susceptible to environmental disturbances. Among other things, this requires shielding of the measuring lines, wherein the distances between the electrodes, i.e. from the glass electrode to the reference electrode, and from the electrodes to the measuring device are to be minimized.
Another disadvantage of the glass electrodes is that there is the danger of them releasing sharp-edged glass splinters when breaking, so that the employment of glass electrodes is limited in certain areas, such as in the food industry.
In contrast to the glass electrodes, the usage of ion-sensitive field effect transistors represents a break-proof alternative to conventional glass electrodes. Furthermore, these sensors are suitable for the miniaturization of the measuring system, the production of integrated systems, and low-cost manufacturing, so that the ion-sensitive field effect transistor is superior to the conventional glass electrode with respect to the above aspects.
The application of ion-sensitive field effect transistors for the ionic concentration measurement and particularly for the measurement of the pH value in aqueous media has been known for a long time, and is described, for example, in the document Bergveld P., IEEE Trans. Biomed. E 17 (1970) 70. For producing hydrogen ion-sensitive layers of the ion-sensitive field effect transistors, various materials, such as SiO2, Si3N4, Al2O3, AlxSiyOz, ZrO2, Ta2O5 and diamond-like carbon (DLC) have already been examined and described. In this respect, see the documents Van der Schoot et al., Sensors & Actuators 4 (1983), 267; Sobczynska D. et al., Sensors & Actuators 6 (1984), 93; Klein M. et al., VDI-Berichte 509 (1983), 275; Sakai T. et al., Internat. Electron Devices Meeting, Techn. Digest (1987), 711; Abe H. et al., IEEE Trans. Electron. Dev. ED-26 (1979), 1939. The introduction of the metal oxide layers achieved significant improvements of the sensor properties as compared to the ion-sensitive field effect transistors with a sensitive layer of Si3N4, particularly with respect to pH sensitivity, pH sensor linearity, pH selectivity, i.e. cross-sensitivity to other than the H+ ion to be measured, hysteresis behavior, response behavior, start-up time, photosensitivity, drift and long-term stability. The most advantageous sensor properties of ion-sensitive field effect transistors are known to have been achieved with a sensitive layer of Ta2O5.
However, the usage of metal oxides as sensitive layer has the disadvantage that, due to the crystalline structure of the metal oxides after the necessary annealing processes, the settling times of these sensors when changing the measured solution is larger than with the ion-sensitive field effect transistors with the normally amorphous Si3N4 layers.
Without sufficient annealing processes, the metal oxide layers do not achieve the advantages over the sensitive layers of Si3N4. A further improvement in the sensor stability of ion-sensitive field effect transistors in aggressive media, such as bases and hydrofluoric acids, is ascribed to the carbon-based layers, wherein, however, their response behavior is less favorable than with a sensitive layer of Si3N4.
The areas of application of the above ion-sensitive field effect transistors with metal oxide layers are mostly restricted to limited pH ranges and temperature ranges of the solutions. Therefore, there has been no consistent introduction of pH measurements on the basis of ion-sensitive field effect transistors in the industrial process measurement technology or in environment monitoring yet, so that there has been no addition to or substitution of the glass electrodes in these areas.
It is particularly to be mentioned in this respect that all above layer materials are single component systems, i.e. they are simple, pure compounds limited in acidity with respect to their single pH sensitive component, e.g. [TaOH] in the case of Ta2O5, after the hydrolysis of the surface in the solution. This means that the single component systems become non-linear in basic or acid solutions, because there is only one defined pKs and one pHPZC. The limited acidity is further the reason for irreversible damages in the sensor layer at high temperatures. In the case of the glass electrode, the sensor-active layer is an SiO2-based glass with a slight addition of various metal oxides, which both linearize the acidity of the [SiOH] better and increase the stability with respect to bases and acids than would be the case with a pure semi-metal oxide SiO2.
The necessity of a glass structure of the sensitive layer does not exist for the ion-sensitive field effect transistor. That is why the properties of the glass structural substance SiO2 that are disadvantageous with respect to the sensor can be avoided in the same. Such disadvantageous properties are the changing sensor properties by solutions containing hydrofluoric acid and the cross-sensitivity to alkali ions, particularly Na+ ions (see, for example, Bergveld P., IEEE Trans. Biomed. E 17 (1970), 70). Then, a lower pH value is indicated in basic solutions with Na+ ions than would correspond to the real H+ ion concentration. Particularly at elevated temperatures, the pH sensor properties irreversibly deteriorate more and more in these basic solutions and finally the lifetime is limited, because SiO2 is dissolved in strong alkaline solutions. The lifetime is further limited by the etching in solutions containing hydrofluoric acid. In practice, the metal oxide additions are changed to either increase the stability in aggressive media or to reduce cross-sensitivity, wherein, however, the structural substance SiO2 limits the adjustment possibilities, so that only a limited improvement of the electrode is possible.
Furthermore, cross-sensitivities to alkali ions may also occur at surfaces of metal silicates, which may be more pronounced in the case of aluminosilicates AlxSiyOz than in purely thermal SiO2 (see, for example, Abe H. et al., IEEE Trans. Electron. Dev. ED-26 (1979), 1939, or Esashi M., Matsuo T., IEEE Trans. Biomed. BME-25 (1978), 184). It is further known, see, for example, Leistiko O., Physica Scripta 18 (1978), 445, that pure thermal SiO2 has a lower cross-sensitivity to Na+ and K+ ions in ion-sensitive field effect transistors, wherein, however, there is a strong hysteresis of ±0.2 pH and a considerably lower transconductance of about 30 mV/pH as compared to a glass electrode with >58 mV/pH. The document Bergveld P., IEEE Trans. Biomed. E 17 (1970) 70, observed an increased cross-sensitivity to Na+ ions, which was due to an A1 gate on the SiO2 having been etched away after the CMOS process. Even minor temperings at about 400° C. in forming gas annealing are sufficient to form monolayers of aluminosilicate on the SiO2, which considerably change the pH sensor properties. By forming the aluminosilicate, the chemical stability could be increased, which, however, resulted in a decrease of the pH sensitivity.
Another disadvantage of pure metal oxide compounds is that their molecular structure is only adjustable to a limited extent by temperature-time processes. High temperatures after the layer deposition result in the necessary consolidation and freedom of structural defects of the metal oxide and thus in a good chemical stability of the layer, but also in crystallization and thus formation of pronounced disturbances of the surface which may reach all the way to the substrate. However, it is required for a good property of the sensor that the sensor surface is homogeneous, smooth and closed.